MacBook Pro - Macbook Upgrade Questions/Problem

How much faster really are the Intel Core 2 Duos vs. the current ones? Would it be worth selling my current laptop (MB Pro 1.83ghz) and using the money to get a new-new one. (When they come out) Granted I don't do anything really high-end, but it's always nice to have options. While im on the topic of selling this computer, how much do you think I could get for it once the new ones come out.

Problem:

I noticed once I used my friend's MB pro that mine makes an odd noise, all the time. Imagine a fan with a small brush attatched to one of the blades and that little brush keeps hitting something as it goes around. That's what it sounds like, very faint, but very clear. The noise stops when the computer goes to sleep and doesn't get louder with heavy use. Any ideas? (I have applecare, so sending it back is fair game, though im not sure i can afford to be without it for a week..2 weeks, whatever it is.)